1. Field of the Invention
The present invention relates to the field of computer assisted surgery (CAS), in particular procedures for implantation and revision of artificial joint and bone components. Particular focus of the invention is on intraoperative registration of a bone, transfer of preoperatively planned geometry to bone, and intraoperative characterization of the bone. The present invention is particularly adapted for computer assisted Femoral Head Resurfacing (FHR).
2. Background Art
With an aging population, bone and joint deterioration due to a number of different diseases—most notably arthritis, is an increasingly common occurrence. A common procedure—total hip replacement (total hip arthroplasty—THA)—has been successfully used in older patients with predictable and durable results. Some of the initial issues in regards to implant mobility and wear and tear have been overcome with the advent of new generation of implants and materials. However, the results in younger and more active patients have been less predictable, especially where older reconstructive techniques and materials such as bone cement have been used. In THA procedures involving younger patients a significant amount of problems have been observed in relation to the articular bearing wear and component loosening [Primary uncemented Harris-Galante acetabular components in patients 50 years old or younger: results at 10 to 12 years, by Duffy G. P., Prpa B., Rowland C. M., and Berry D. J., Clin Orthop Relat Res, 2004 October; Why knees fail: lessons learned, by Callaghan J. J., O'rourke M. R. and Saleh K. J., J Arthroplasty, 2004 June; Total hip replacement in patients younger than thirty years old. A five-year follow-up study, J Bone Joint Surg Am, 1981 December; Comparison of primary total hip replacements performed with a standard incision or a mini-incision, by Dorr, L. D., J Bone Joint Surg Am, 2005 March; and Long-term results of Charnley low-friction arthroplasty in young patients, by Joshi A. B., Porter M. L., Trail I. A., Hunt L. P., Murphy J. C., Hardinge K., J Bone Joint Surg Br, 1993 July]. While some of the issues have been overcome, there is still a significant concern in relation to the long term prognosis for active patients whose remaining lifespan may be in excess of 150 years.
A particular concern for younger patients is the desire to “gain time” and delay entry to the medullary canal—the inside cavity of the bone which contains the bone marrow. THA procedures require a long stem to be inserted into the medullary canal, which can trigger fat emboli during surgery, potentially leading to pulmonary occlusion, and can begin a process of bone resorption and degeneration which can limit the overall life of the implant. An alternative procedure to THA, available in modern orthopedic surgery, is Femoral Head Resurfacing (FHR). FHR has been particularly helpful for younger patients afflicted with osteoarthritis or avascular necrosis of the femoral head [Metal-on-metal hybrid surface arthroplasty: two to six-year follow-up study, by Amstutz H. C., Beaule P. E., Dorey F. J., Le Duff M. J., Campbell P. A., and Gruen T. A., J Bone Joint Surg Am, 2004 January; Surface arthroplasty for osteonecrosis of the hip: hemiresurfacing versus metal-on-metal hybrid resurfacing, by Beaule P. E., Amstutz H. C., Le Duff M., Dorey F., J Arthroplasty, 2004 December; and Metal-on-metal resurfacing of the hip in patients under the age of 55 years with osteoarthritis, by Daniel J., Pynsent P. B., McMinn D. J., J Bone Joint Surg Br, 2004 March]. In FHR method, the acetabular cavity of the innominate is lined with a new socket similar to the THA. However, unlike THA which requires cutting off superior component of the femur at the neck point, FHR requires only that the femoral head is reamed such that a new artificial femoral head cup can be secured over it. FHR technique not only preserves femoral head bone stock, but, in comparison to THA, also more closely approximates normal hip kinematics, joint stability and proprioception, while minimizing the potential for post-operative leg length discrepancy and stress shielding of the proximal femur [Birmingham hip resurfacing arthroplasty. A minimum follow-up of five years, by Treacy R. B., McBryde C. W. and Pynsent P. B. J Bone Joint Surg Br, 2005 February; Metal on metal surface replacement of the hip. Experience of the McMinn prothesis, by McMinn D., Treacy R., Lin K. and Pynsent P., Clin Orthop Relat Res, 1996 August; and Surface arthroplasty for osteonecrosis of the hip: hemiresurfacing versus metal-on-metal hybrid resurfacing, by Beaule P. E., Amstutz H. C., Le Duff M., Dorey F., J Arthroplasty, 2004 December]. Furthermore, it has been suggested that due to extremely low rates of articular bearing wear in metal on metal hip resurfacing arthroplasty, this technique may permit the patient to return to a greater level of activity and sport than other reconstructive options [Surface arthroplasty for osteonecrosis of the hip: hemiresurfacing versus metal-on-metal hybrid resurfacing, by Beaule P. E., Amstutz H. C., Le Duff M., Dorey F., J Arthroplasty, 2004 December]. Indeed, even though younger patients place high activity demands on these implants, early clinical results are very good with multiple authors reporting 2 to 5 year implant success of 194-198% [Metal-on-metal hybrid surface arthroplasty: two to six-year follow-up study, by Amstutz H. C., Beaule P. E., Dorey F. J., Le Duff M. J., Campbell P. A., and Gruen T. A., J Bone Joint Surg Am, 2004 January; Metal-on-metal resurfacing of the hip in patients under the age of 55 years with osteoarthritis, by Daniel J., Pynsent P. B., McMinn D. J., J Bone Joint Surg Br, 2004 March; and Birmingham hip resurfacing arthroplasty. A minimum follow-up of five years, by Treacy R. B., McBryde C. W. and Pynsent P. B. J Bone Joint Surg Br, 2005 February].
Despite the attractive clinical results and success, however, there continues to be a number of complications and potential concerns with the surgical technique that are unique to FHR. In comparison to total hip arthroplasty, the surgical technique is more complex and demands higher degree of precision from the surgeon. In order to adequately visualize the femoral head and neck to properly orient and place the new cup, surgeon must use a wide exposure of the hip which results in longer incision and potentially longer healing times. The instrumentation for femoral head preparation is based entirely upon the placement of a femoral head and neck guide pin. Pin placement is a time-consuming process in the operating room and despite the surgeon's efforts to centre the pin using intraoperative guides and calipers, the surgical technique can be unreliable [Variability of femoral positioning in hip resurfacing arthroplasty, 51st Annual Meeting of the Orthopaedic, by Shekhman M., Masri B. A., Greidanus N. V., Garbuz D. S., Duncan C. P., Anglin C., Hodgson A. J., and Inkpen K. B., Research Society, Washington, D.C., Feb. 20-23, 2005] and may result in sub-optimal component placement or notching. Notching—unplanned cortical bone violation—occurs where the bone is reamed or cut in a location where it will not be covered by the new femoral head cup. Notching is of particular concern as it has been demonstrated to increase the risk of postoperative femoral neck fracture [Metal-on-metal resurfacing of the hip in patients under the age of 55 years with osteoarthritis, by Daniel J., Pynsent P. B., McMinn D. J., J Bone Joint Surg Br, 2004 March; and Metal on metal surface replacement of the hip. Experience of the McMinn prothesis, by McMinn D., Treacy R., Lin K. and Pynsent P., Clin Orthop Relat Res, 1996 August]. In addition, malposition of the guide pin may result in oversizing the femoral head component, which may then necessitate excessive removal of acetabular bone stock for acetabular component placement. Malposition of the femoral component may also compromise the hip's range of motion and patient satisfaction with the procedure. Failure to pay attention to these nuances may explain some of the unique complications of resurfacing arthroplasty which include post-operative femoral neck fracture requiring revision surgery, implant loosening, and avascular necrosis of the femoral head, which may result in symptoms of pain or femoral head collapse and loosening of the resurfacing component [Surface arthroplasty for osteonecrosis of the hip: hemiresurfacing versus metal-on-metal hybrid resurfacing, by Beaule P. E., Amstutz H. C., Le Duff M., Dorey F., J Arthroplasty, 2004 December]. These highly technical aspects of the procedure are evident in improvement in the accuracy of component placement and reductions in the rates of femoral neck notching, fracture and revision with increasing numbers of resurfacing procedures performed [Outcomes of limited femoral resurfacing arthroplasty compared with total hip arthroplasty for osteonecrosis of the femoral head, by Mont M. A., Rajadhyaksha A. D. and Hungerford D. S. J Arthroplasty, 2001 December; and The results of metal on metal resurfacing hip arthroplasty: learning curve stratification of results, by Mont M., Bezweda H., Thomas C., Etienne G., American Academy of Orthopaedic Surgeons, Washington D.C., Feb. 22-27, 2005].
Existing methods to improve femoral head/neck pin placement and instrumentation of the femoral head have largely involved preoperative planning, complex kinematic modelling, and radiographic calculations. In addition, a number of mechanical apparatus such as jigs, calipers, and femoral neck gauges have been developed to help guide the surgeon's pin placement intraoperatively. Despite the refinement of these methods, there continues to be tremendous variability in final guide-pin placement and femoral component placement [Variability of femoral positioning in hip resurfacing arthroplasty, 51st Annual Meeting of the Orthopaedic, by Shekhman M., Masri B. A., Greidanus N. V., Garbuz D. S., Duncan C. P., Anglin C., Hodgson A. J., and Inkpen K. B., Research Society, Washington, D.C., Feb. 20-23, 2005] among both novice and experienced hip surgeons.